Re: DNS Reverse Shell sig

[James Lay <jlay () slave-tothe-box net>]

On 2015-02-04 02:06 PM, Dave Killion wrote: 

> The initial letter
before the dash is part of the "label" (that 0x3A is measuring) so yes -
the fact you're counting 58 bytes total (between the character class,
the dash, and your pcre string) means you'll match on 0x3A all the time.

> In fact, you could probably remove the last pcre check entirely,
since doing that 0x3A check will perform the byte-count for you... 
>
-Dave
> On Wed Feb 04 2015 at 1:02:37 PM James Lay
<jlay () slave-tothe-box net [9]> wrote:
> > On 2015-02-04 01:29 PM,
rmkml wrote:
> > > Thx James for sharing,
> > >
> > > Length is always the
same, add 0x3A (dns length) like this ?
> > > Add "-" for better
performance ?
> > > modify pcre to use relative?
> > alert udp
$HOME_NET any -> any 53 (msg:"Possible Python Reverse DNS
> > Shell";
content:"|01 00 00 01 00 00 00 00 00 00 3A|"; depth:11;
> > offset:2;
fast_pattern; content:"-"; within:1; distance:1;
> >
pcre:"/^[a-z0-9]{56}/Ri";
> >
reference:url,lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html
[1];
> > classtype:bad-unknown; sid:10000150; rev:2;)
> > > Best
Regards
> > > @Rmkml
> > >
> > >
> > > On Wed, 4 Feb 2015, James Lay
wrote:
> > > > In my testing of
http://lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html [2]
I
> > > > noticed that during the reverse shell session a semi-constant
showed
> > > > up...namely a character followed by a dash, followed by 56
other
> > > > characters. Pretty sure this could be changed in the python
code, but
> > > > this will catch this in it's current form. It will not
fire on each
> > > > and every dns query, but will most likely fire at
least during the
> > > > session.
> > > >
> > > > alert udp $HOME_NET any ->
any 53 (msg:"Possible Python Reverse DNS
> > > > Shell"; content:"|01 00
00 01 00 00 00 00 00 00|"; depth:10; offset:2;
> > > > fast_pattern;
pcre:"/[a-z]-[a-z0-9]{56}/i";
> > > >
reference:url,lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html
[3];
> > > > classtype:bad-unknown; sid:10000150; rev:1;)
This WILL most likely FP if you're looking for a domain that matches
> > the above style, but I couldn't find any such domain in my logs.
> >
> > Enjoy.
> > > > James
> >
> > That's a good idea..would that still
catch the initial "[a-zA-Z]-" at
> > the start though? Or just the
"-[a-z0-9]{56}"...thanks RM!
> > James
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Awesome....thanks for the education on
this! 

James 

Links:
------
[1]
http://lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html
[2]
http://lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html
[3]
http://lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html
[4]
http://goparallel.sourceforge.net/
[5]
mailto:Snort-sigs () lists sourceforge net
[6]
https://lists.sourceforge.net/lists/listinfo/snort-sigs
[7]
http://www.snort.org
[8] http://blog.snort.org
[9]
mailto:jlay () slave-tothe-box net

------------------------------------------------------------------------------
Dive into the World of Parallel Programming. The Go Parallel Website,
sponsored by Intel and developed in partnership with Slashdot Media, is your
hub for all things parallel software development, from weekly thought
leadership blogs to news, videos, case studies, tutorials and more. Take a
look and join the conversation now. http://goparallel.sourceforge.net/

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